Steroid hormones are known to have important effects on behavioral and morphological characters that differ between the sexes. Many sexual dimorphisms are often first expressed coincident with increases in circulating androgens seen at male sexual maturity, as is the case for deepening of the voice in human males. In some species, however, some sexually mature males regularly fail to express male-typical morphology or behavior. Species with such clear intra-sexual dimorphisms in addition to the more typical inter-sexual dimorphisms present ideal model systems for examining neuroendocrine mechanisms underlying both l) normal development of sexually dimorphic characters and 2) uncoupling of male sexual maturation and the expression of male-typical characters. The fish, Porichthys notatus, is ideally suited for such studies because there are distinct inter- and intra- sexual dimorphisms of its simple vocal system. This proposal aims to determine l) the normal development of the vocal system in juveniles, and 2) females and the two types of males differ in circulating levels of particular steroid hormones during development or if system components are differentially sensitive to circulating androgens as a result of differences in androgen receptor expression. The existence of reproductively mature and competent males that are feminized/demasculinized with respect to both neural and muscular components of the vocal system provides an ideal opportunity to broaden our understanding of how the expression of sexual dimorphisms can be uncoupled from sexual maturation in species, including humans, where intra-sexual variation is more continuous.